Need Major Help with Canal Lock Problem!

As the title says, I need major help with a problem for my AP Physics class and I have to explain my answer orally to the class. Here is the question and if you could, could you answer it and follow up with an explanation? Any help would be greatly appreciated. Here it is:

A boat in a canal lock sinks: does the water level rise, sink, or remain the same? Explain!

try this: take a container full of water and then add something to it (like a toy boat perhaps?) and slowly submerge it. You will then get your answer to whether it sinks, floats or stays the same. If you are still having trouble describing why it happens, post back.

I have this - okay a canal lock is something that helps the boat travel through a dramatic change in the riverbed, that causes say a small waterfall - in this case that waterfall is surrounded by 2 gates
as a boat approaches the lock it (canal lock) fills up the water to match the waterlevel the boat is in right now. As it reaches that level, the gate opens and boat goes through. Then lock closes and waterlevel starts to drop - to match the 2nd waterlevel. As the required level is reached 2nd gate opens and "Voila!" the boat has just traveled through a lock, which acts in a way as an elevator.